解决 ‘Could not fetch URL https://pypi.python.org’的问题

python pip 安装出现pypi 源找不到的问题，是因为pypi 更改了官方网址
Could not fetch URL https://pypi.python.org/simple/pyopenssl/:
There was a problem confirming the ssl certificate: [SSL: TLSV1_ALERT_PROTOCOL_VERSION] tlsv1 alert protocol version (_ssl.c:590) - skipping
Could not find a version that satisfies the requirement pyopenssl (from versions: )
No matching distribution found for pyopenssl

因为源无法登陆，所以其实只要换一个国内的pypi源就好了
例如换成豆瓣的

// An highlighted block
pip install xlrd -i http://pypi.douban.com/simple --trusted-host pypi.douban.com